1. Field of the Invention
This invention relates to an apparatus and method for communicating information using fractional bits-per-symbol signaling rates responsive to communication channel conditions.
2. Description of the Relevant Art
There exist applications for which it is desirable to transmit transmission symbols that each is composed of a number of information bits which may not represent an integer. The signal constellations associated with such transmissions symbols, then, corresponds to non-power-of-two constellation sizes and/or non-integer constellation sizes.
What is needed is a device that affords fractional bits per symbol digital communication approaching a maximum allowable bit rate yet does so in an efficient manner permitting use of compact code sets, or use of a partial response receiver. Before proceeding with a description of exemplary embodiments, it should be noted that the various digital signaling concepts described hereinxe2x80x94with the exception, of course, of the inventive concept itselfxe2x80x94are all well known in, for example, the digital radio and voiceband data transmission (modem) arts and thus need not be described in detail herein. These include such concepts as multidimensional signaling using 2N-dimensional channel symbol constellations, where N is some integer; trellis coding; fractional coding; scrambling; passband shaping; equalization; partial response; Viterbi, or maximum-likelihood, decoding; Quadrature Amplitude Modulation(QAM); Orthogonal Frequency Division Multiplexing (OFDM),etc.
The invention herein provides communication devices affording non-integer bits per symbol digital communication at bit rates approaching optimality for a given set of constraints, using a compact code set or utilizing partial response receiver. In general, the devices which embody the present invention, can include a transmitter, a receiver, or both. These devices manipulate arriving data from a data bit form to a transmission symbol form in a data transformer, advantageously using knowledge of one or more data channel conditions to dynamically and continuously adjust the constellations used to represent the transmitted data. Successive transmission symbols each can contain a varying selectable predetermined integer number of data bits, as governed by a constellation selection controller that is connected with the data transformer. Successive symbols can be transmitted at different time stamps or at different frequency locations. A performance metric estimate can be used to determine which constellation is to be used. A receiver using a sequence estimation technique can optimally decode the received signals given the condition that (i) the soft-decision symbols at the receiver are correlated through the employed channel coding and/or through the introduction of defined partial channel response, and (ii) there exists knowledge of the signal-to-noise ratio (SNR) metric of each soft-decision symbol. Having these techniques, a transmitter, for example, can transmit on the average non-integer information bits per symbol, say between k and k+1, with the multiplexing of a first constellation, representing a first selectable predetermined integer number of data bits, e.g., k bits per symbol, and a second constellation, representing a second selectable predetermined integer number of data bits, e.g., k+1 bits per symbol, such that the desired non-integer information-bit-per-symbol transmission is achieved. It is desired that such multiplexing be done continuously and dynamically. Furthermore, the digital communication facilitated by the invention herein is not limited to temporal sequence transmission (i.e., the time domain) but also can be used in the frequency domain, or both.